To understand if the decrease of the friction coefficient was, indeed, associated with the atmosphere, a test (5 Hz) was performed under an inert argon atmosphere (orange curve). Indeed, the observed friction coefficient was higher than those obtained under the refrigerant gas atmosphere, and it has a tribological behaviour similar to the one observed under ambient air.
Analyzing the average friction coefficient during the steady-state for each testing condition presented in Figure 11, it is clear that the refrigerant gas atmosphere induces lower friction coefficients for 5 and 20 Hz frequency.
The presence of tribolayers was verified for the ambient air testing atmosphere. Additionally, significant amounts of debris were observed scattered around the contact region, as shown in Figure 12A. Tribolayers was also found on the testing performed with 5 and 20 Hz under the refrigerant gas atmosphere (Figure 12B). However, significantly less, and in some cases even none, debris was detected surrounding the tribolayers. Moreover, the original topographical features of the surfaces (machining grooves) are still visible, which indicates that the tribological contact does not induce high plastic deformation on the cylinder surface. No tribolayers were observed for tests under argon atmosphere or 40 Hz frequency with refrigerant gas.
Severe plastic deformation is observed in the burnished area of the pistons tested under ambient air atmosphere, as shown in Figure 12C. Therefore, as previously mentioned, the DLC coating was removed due to the tribological action. However, the DLCs tested under R134a atmosphere (5 and 20 Hz) present an integral aspect (Figure 12D).
Figure 13 illustrates typical Raman spectra of the tribolayers on the cylinders. They are similar regardless of the atmosphere (ambient air or refrigerant gas), presenting typical D and G bands. Additionally, the ID/IG ratios are always greater than the unit. The G bands show a dislocated position to a higher Raman shift (1610.00 cm-1 instead of 1555.95 cm-1), as already observed by Salvaro et al. (2017).
This result indicates that the carbon presents itself as a highly disordered and high defect density graphitic structure in the tribolayers (Ferrari and Robertson, 2001; Robertson, 2002; Casiraghi et al., 2005). According to Salvaro et al. (2017), the graphitization of the uppermost layer of the DLC coating associated with DLC debris due to the contact are probably, the leading cause of disorder of the graphite-based tribolayer.
Using WLI, the tribolayers thicknesses were calculated by subtracting the average tribolayers height from the average height of their vicinities height, as demonstrated in Figure 14A. The results for the tests performed under the refrigerant gas atmosphere (5 and 20 Hz frequencies) are equivalent, with an average thickness of around 800 nm for both cases. For the ambient air condition, the average tribolayers thickness is 1.3 um, but the standard deviation is significantly higher (Figure 14B) as a consequence of the gradual formation of a non-continuous and non-homogeneous tribolayer presenting areas containing thinner and less uniform tribolayer whereas other areas show thicker and well-formed tribolayer as described by Barbosa et al. (2015).
Figure 12A shows the EDS analyzes of the tribolayers formed under ambient air. It indicates that they are constituted of iron, carbon, and oxygen, typical elements from cylinder/piston (substrate), coating, and atmosphere, respectively. Therefore, the tribolayers result from wear debris mixed, comminuted, oxidized, and compressed in the contact. Similar results were found on the tribolayers generated under the R134a refrigerant gas atmosphere (Figure 12B), but in this case, a systematic presence of fluorine originated from the R134a was detected. The burnished areas on the DLC piston tested under the refrigerant gas atmosphere also had detectable fluorine, but in low concentration (~1.8%wt), besides carbon (Figure 12D).
According to Wang et al. (2013) and Yu et al. (2003), fluorine can connect itself with carbon’s dangling bonds on the surface (C-F). The tribological work wears out the cylinder and piston (DLC), thus inducing high-disorder, high-defect density graphite (graphitization). During this process, the energy available in the contact is enough to break F-C bonds in the R134a molecule, which leaves fluorine ions free to recombine with the carbon from tribolayers on the cylinder or DLC coating on the piston (5 and 20 Hz), therefore an in situ tribo-fluorination phenomenon.
Wang et al. (2013) have also shown that the polar fluorine-carbon bond on the uppermost layer of the surface results in a “smoother” surface electron density distribution. Thus, repulsive interactions govern the friction coefficient between two surfaces with these features in tribological interaction, as shown in Figure 15.
In other words, in situ tribo-graphitization and tribo-fluorination of carbon structures in the tribolayers and DLC coating present a positive synergy to reduce the friction coefficient of the oil-less hermetic compressor piston-cylinder tribopair.
In this study, a proprietary tribological emulator was used to test a piston-cylinder tribopair from an oil-less hermetic compressor. DLC coated piston against ¼ stainless steel cylinder were tested under close-to-real conditions, including the atmosphere (R134a). Additionally, tests with ambient and inert atmospheres were performed to evaluate the effect of these parameters on the tribological performance. The following conclusions were reached.
- The friction coefficients are four times smaller when using a fluorine-rich testing atmosphere compared to the ambient atmosphere. However, in higher frequencies (40 Hz), the friction coefficient does not stabilize, reaching values over 0.4, and the DLC coating fails.
- Tribolayers found under refrigerant gas atmosphere have similar thicknesses (~800 nm). In contrast, tribolayers formed under ambient air conditions have scattered thicknesses.
- The friction coefficient is governed by the formation of tribolayers on the cylinders. All tribolayers found contain Fe, O, and C. The carbon is present as a highly disordered and highly defects-dense graphitic structure. Under refrigerant gas atmosphere, carbon dangling bonds on the uppermost tribolayers surface and DLC coating are completed by fluorine from the refrigerant atmosphere, resulting in a repulsive interaction between the surfaces, consequently inducing low coefficients of friction (~0.04).
- In situ tribo-fluorination of carbon structures in the tribolayers and DLC is feasible and represents a new, cost-effective and way to control friction and wear on -less hermetic compressor piston-cylinder tribopair.
In the present work, we overview the multidisciplinary development of a regular, lubricated, hermetic compressor that works in an on-off cycle, circular motion, single-speed, many tribological contacts, into an innovative, linear motion, variable displacement, single tribological contact, oil-less hermetic compressor presenting high versatility in terms of refrigerator design, sustainability and improved efficiency. The original approach encompassed the development of new surface engineering procedures applying purpose-oriented phases to soft substrates. Different types of multi-layers, their thickness, substrate material, processing routes, etc., have been studied and optimized in this context. The results clearly show that the tribological behaviour of carbon derived coatings is strongly influenced by the surrounding atmosphere. Si-rich hydrogenated DLC (a:C-H) presented enhanced tribological properties when tested under fluorine-rich atmospheres demonstrating the feasibility of the in situ tribo-fluorination of carbon structures, which represents a new, cost-effective and efficient way to control friction and wear on oil-free hermetic compressor piston-cylinder tribopair.
Data Availability Statement
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.
GB was the main investigator regarding in situ fluorination, supervised the obtainment of the samples, supervised all the experiments, described and co-analyzed the results, and wrote the first short draft of the article. DS helped supervise the research and contributed to analyzing the results, particularly those related to tribo-tests, and helped write the first short draft of the article. CB was responsible for managing the various phases of the project, helped supervise the research, and contributed to analyzing the results. RB helped design the whole project, actively participated in the management, fundraising, and analysis of the results. AK helped design the whole project, its management, and supervision, to analyze the results and financing of the different stages. JM designed the entire project, particularly the tribological aspects, supervised the research, helped to analyze the results, and had a major role in the conception and writing of the final manuscript. All authors contributed to the article and approved the submitted version.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The authors acknowledge the agencies for funding this research: Fulbright Commission (USA), CNPq, FAPEMIG, BNDES, CAPES-PROEX (Brazil) as well as Nidec Global Appliance/Embraco. Special thanks are due to their co-authors: M. Sc. M.V. Barbosa, UFSC; Dr. T. Bendo, UFSC; Prof. H.L. Costa, UFU; Dr. N. Demas, Argone National Laboratory; Dr. R.O. Giacomelli, UFSC; Dr. G. Hammes, UFSC; M. Sc. E.R. Hulse, Nidec Global Appliance/Embraco; M. Sc. T.S. Lamim, UFSC; Dr. L.O.C. Lara, UFES; Prof. A. Polycarpou, Texas A&M; Dr. M.B dos Santos, UFU; Dr. P.H.T. Shioga, UFSC; M. Sc. M. Silverio, Nidec Global Appliance/Embraco; M. Sc. P.B Soprano, Nidec Global Appliance/Embraco.
Gabriel Borges (GB) is a Student / Intern at Federal University of Santa Catarina, Florianopolis, Brazil.
Diego Salvaro (DS) is a Researcher at Federal University of Santa Catarina, Florianopolis, Brazil.
Roberto Binder (RB) is a Senior Researcher at Embraco (Brazil), Joinville, Brazil.
Cristiano Binder (CB) is (Primary) an Adjunct Professor at Federal University of Santa Catarina, Florianopolis, Brazil and an Adjunct Professor at Materials Laboratory – LabMat: Federal University of Santa Catarina, Florianópolis-SC, Brazil.
Aloisio N. Klein (AK) is a Professor at Federal University of Santa Catarina, Florianopolis, Brazil.
Jose D. B. de Mello (JM) is a Professor, Federal University of Uberlandia, Uberlândia, Brazil.
Copyright © 2021 Borges, Salvaro, Binder, Binder, Klein and de Mello.
Correspondence: Jose D. B. de Mello, email@example.com
The original article was published in www.frontiersin.org